1. Field of the Invention
This invention relates to apparatus providing protection in multiwire branch circuits of electric power distribution systems, and in particular, to circuit breakers providing protection from arcing faults and ground faults in such circuits.
2. Background Information
Branch circuits in electric power distribution systems often share a common neutral conductor to reduce the wiring required. Such multiwire branch circuits are often referred to as "home runs." Home runs are only permitted under certain conditions. Generally, they are permitted as long as the two line conductors are energized by separate phases or by a center tapped single phase to avoid overloading the neutral conductor, and as long as all ungrounded conductors are disconnected simultaneously at the panel board where the branch circuits originate. This simultaneous disconnection of the ungrounded conductors can be accomplished with a two pole disconnect, two single pole circuit breakers with a handle tie, or a two pole circuit breaker.
Presently, such multiwire branch circuits are provided with short circuit and overcurrent protection by the tied single pole breakers or the two pole breaker. Only the two pole breaker can also provide ground fault protection by the addition of a common ground fault detector.
Recently, there has been an increased interest in providing protection from arc faults. Arc faults are intermittent high impedance faults which can be caused for instance by worn insulation, loose connections, broken conductors, and the like. Because of their intermittent and high impedance nature, they do not generate currents of sufficient instantaneous magnitude or sufficient average current to trigger the thermal-magnetic trip device which provides the short circuit and overcurrent protection. Various types of arc fault detectors have been proposed, but to my knowledge they have not been adapted to multiwire branches.
Parent application Ser. No. 08/939,263 filed on Sept. 23, 1998 discloses apparatus for detecting faults in multiwire branch circuits where both poles are tripped simultaneously in response to all faults, overcurrent, arcing and ground faults, regardless of which line conductor is involved. This arrangement complies with the general conditions for multiwire branch circuits discussed above where all ungrounded conductors must be opened simultaneously. This is because the two line conductors can be connected to a single appliance, such as an electric stove for instance, to provide 220 volts, or to two separate outlets in a common receptacle. However, where the two line conductors, with the common neutral, are connected to independent loads, both circuits do not have to be interrupted simultaneously in response to an overcurrent condition (a short circuit or overload) in one line conductor. By only interrupting current in the affected line conductor, disruption of service is minimized. On the other hand, an arcing fault occurs because of a wiring failure. This wiring failure may be in the multiwire cable, and therefore, both poles should be tripped.
There is a need for a circuit breaker which can provide arc fault protection as well as overcurrent, and ground fault protection for multiwire branch circuits and which isolates overcurrent trips to the affected circuit while tripping both circuits in response to arc faults or ground faults.